Various types of fluids can be used in the oil and gas industry. Non-limiting examples include drilling muds, cements, gravel slurries, and stimulation treating fluids. Such fluids can be pumped into oil or gas wells in ways known in the industry. It is desirable to know the various characteristics of these fluids to determine how they will act while being pumped and placed in, or circulated through, the wells.
Rheology is the branch of physics dealing with the deformation and flow of matter. Viscosity, elasticity, and consistency are rheological characteristics that sometimes need to be measured for a given fluid. Such rheological characteristics can be non-linear functions of variables such as time, temperature, and pressure. Known devices used to test fluids for these characteristics include viscometers, rheometers, and consistometers.
In the oil and gas industry fluids containing suspended particulate can be used. These fluids can be in a gel form to better support the particulate. The particulate is typically referred to as proppant, and consists of sized particles mixed within a fluid, such as a fracturing fluid. In addition to naturally occurring sand grains, man-made or specially engineered proppants, such as resin-coated sand, glass beads, or high-strength ceramic materials like sintered bauxite, may also be used. Proppant materials can be sorted for size and sphericity to provide an efficient conduit for production of fluid from the reservoir to the wellbore once placed in the formation. Proppants can vary in size from powder to sandy.
The treatment of subterranean formations penetrated by a well bore to stimulate the production of hydrocarbons or the ability of the formation to accept injected fluids has long been known in the art. One of the most common methods of increasing productivity of a hydrocarbon-bearing formation is to subject the formation to a fracturing treatment. This treatment is effected by injecting a gas, liquid, or two-phase fluid down the well bore at sufficient pressure and flow rate to fracture the subterranean formation. Continued pumping of the fracturing fluid containing proppant into the fracture results in placement of the proppant within the fracture. Following the treatment, the fracturing fluid is recovered from the well bore or permitted to migrate from the fracture leaving the propping agent remaining in the fracture. The propping agent prevents the complete closure of the fracture to provide a flow channel through which an increased quantity of a hydrocarbon or other fluid can flow.
For fluids that carry proppants, it is desirable that the proppant be suspended in the fluid for a length of time before settling. For this reason the fluid is most commonly used in a gelled form. Several factors, such as heat and length of time, will contribute to the loss of suspension, due to the gelled fluid melting or breaking down. The characteristics such as viscosity, elasticity, and consistency of the fluid are important rheological characteristics that contribute to the suspension of the particulate in the fluid, and these characteristics, and the rate of degeneration of these characteristics, must be measured in these fluids.
A typical fluid used to transport proppant has a viscosity that changes during the time the fluid is used in a well. Viscosity is defined as the ratio of shear stress to shear rate (velocity gradient). If this ratio changes with shear rate, this may be referred to as “apparent viscosity function.” Viscosity is one parameter of the fluid that defines the fluid's ability to support the proppant in suspension. However, to measure a single viscosity point or the apparent viscosity function does not directly indicate the time during which the fluid will support proppant in suspension and the time during which the fluid will not. That is, a measurement that merely shows a changing viscosity does not indicate when the particulate is in suspension within the fluid and when it is not (i.e., when the proppant has settled out of the fluid). Thus, there is the need for a device and method which can test fluids to determine times during which proppant is suspended in the fluid and times during which proppant settles out of suspension. There is the more particular need for a device and method to measure the viscous and elastic properties of a fluid, both with and without proppant, under dynamic conditions at elevated temperatures and pressures at a variety of shear rates and in such a way as to directly indicate particle transport, suspension and settling. At least one embodiment of such a device and method desirably should also be suitable for use at a well site to measure crosslink time of a fluid being pumped into the well.